# 946. Validate Stack Sequences

#### Medium

***

Given two integer arrays `pushed` and `popped` each with distinct values, return `true` *if this could have been the result of a sequence of push and pop operations on an initially empty stack, or* `false` *otherwise.*

 

**Example 1:**

```
Input: pushed = [1,2,3,4,5], popped = [4,5,3,2,1]
Output: true
Explanation: We might do the following sequence:
push(1), push(2), push(3), push(4),
pop() -> 4,
push(5),
pop() -> 5, pop() -> 3, pop() -> 2, pop() -> 1
```

**Example 2:**

```
Input: pushed = [1,2,3,4,5], popped = [4,3,5,1,2]
Output: false
Explanation: 1 cannot be popped before 2.
```

 

**Constraints:**

* `1 <= pushed.length <= 1000`
* `0 <= pushed[i] <= 1000`
* All the elements of `pushed` are **unique**.
* `popped.length == pushed.length`
* `popped` is a permutation of `pushed`.

```python
class Solution:
    def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
        stack = []
        push_index = 0
        pop_index = 0
        while push_index < len(pushed) and pop_index < len(popped):
            while stack and stack[-1] == popped[pop_index]:
                stack.pop()
                pop_index += 1
            stack.append(pushed[push_index])
            push_index += 1
        while stack and stack[-1] == popped[pop_index]:
            stack.pop()
            pop_index += 1
        return True if len(stack) == 0 else False
            
```